1. Field of the Invention
The invention relates to novel 2-alkylaryl 2-hydroxyiminoacetic acids as well as to a process for their preparation. The 2-hydroxyiminoacetic acids according to the present invention have outstanding properties as metal-extraction agents. The invention therefore also relates to a process for the extraction of metal values with the aid of these 2-hydroxyiminoacetic acids.
2. Description of the Prior Art
One of the processes used for the production of metals from ores comprises grinding the ore and extracting the ground ore with an aqueous solution containing sulphuric acid. The acidic aqueous extract phase thus obtained usually contains a mixture of metal ions. An alkaline material, for example, aqueous ammonia, is added to this extract phase to increase the pH to a value suitable for the selective liquid-liquid extraction of one of the metal values with an extractant comprising an organic solvent and a dissolved organic extraction agent. This organic extraction agent is more soluble in the organic solvent that in water and forms complexes with the metals to be extracted. These complexes are also more soluble in the organic solvent than in water.
The liquid-liquid extraction can be conducted continuously by bringing the acidic aqueous solution into contact with the extractant, preferably by vigorous stirring. Then, the organic phase is separated from the aqueous phase and stripped with an aqueous solution containing a strong mineral acid. The metal values are thus transferred to the aqueous stripping solution as metal salts and can then be recovered therefrom, for example, by crystallization, or as metal by electrolysis, while the organic phase containing the released organic extraction agent is advantageously used again for the extraction of further quantities of metal values.
Organic extraction agents are described in:
(1) U.S. Pat. No. 3,224,873, disclosing the use, as copper-extraction agent, of alpha-hydroxy-oximes of the general formula ##STR2## wherein R.sub.1, R.sub.2 and R.sub.3 can represent any of a variety of organic radicals such as aliphatic and alkylaryl radicals, while R.sub.3 may also represent a hydrogen atom. Table I in the above-named specification shows that at a pH of 2.63, 3.0% of the copper was extracted from the acidic aqueous solution, the logarithm of the distribution coefficient being -1.66.
(2) U.S. Pat. No. 3,428,499, disclosing the use, as copper-extraction agent, of 2-hydroxybenzophenone oximes substituted with (a) saturated or ethylenically unsaturated aliphatic group or groups or the corresponding ether group or groups, in combination with alpha-hydroxy-oximes of the general formula I. The acidic aqueous starting solutions in this case usually have a pH between 1.4 and 2.3. For example, the logarithm of the distribution coefficient for copper at a pH of 1.68 has a value of -0.28.
(3) British Pat. specification No. 1,322,532 disclosing the use, as metal-extraction agent, of a hydroxy-oxime of the general formula EQU A -- C (.dbd.NOH) -- R.sup.4 (II)
in which R.sup.4 represents an aliphatic group and A a cyclic organic group carrying as substituents at least a hydroxyl group at a ring carbon atom in a 2-position -- the number 1 being assigned to the ring carbon atom to which the --C(.dbd.NOH)--R group is attached -- and an organic group. These extraction agents are suitably used for copper extraction from aqueous solutions having a pH between 1 and 1.5 and those in which the group A contains, in addition, one or more electron - attracting substituents, from aqueous solutions having a pH between 0 and 1. For example, for a representative oxime the logarithm of the distribution coefficient for copper at a pH of 1.04 has a value of 0.14. The oximes of the general formula II cannot be used for the separation of nickel (II) from cobalt (II), because the complexes of cobalt are difficult to decompose.
(4) German Offenlegungsschrift No. 2,234,901, discloses the use, as metal-extraction agent, of certain salicylaldoximes. The acidic aqueous solutions used in Examples 1 and 3 of this publication had a pH of 2 and 1.49, respectively. From the results obtained logarithms of distribution coefficients for copper of 0.14 and -0.1, respectively, can be calculated.
(5) British Pat. specification No. 959,813, disclosing the use of certain alkanemonocarboxylic acids for the separation of nickel from cobalt present in aqueous solutions having a pH between 7.5 and 9.5; copper is advantageously extracted from aqueous solutions having a pH between 5 and 6.
The rate of extraction attained with the organic extraction agents mentioned under (1), (2), (3), and (4) is too low to justify their application in a column contactor such as a rotating disc contactor (disclosed in British Pat. specification No. 659,241). They call for the use of an impeller mixer. The rate of extraction attained with the organic extraction agents mentioned under (5) is sufficiently high to justify their application in a column contactor, but they cannot be used for the separation of nickel from cobalt present in acidic aqueous solutions.
The present invention provides novel metal-extraction agents, the advantages of which are illustrated hereinafter.